1. Field of the Invention
The present invention relates to a check valve for fluid containers, shock absorbing containers and so on (referred to fluid containers hereinafter), and more particularly to a check valve which is formed by subjecting opposing and walls of a flattened cylindrical body composed of a synthetic resin film or sheet to heat sealing to form a pair of heat sealed portions each comprising curved heat seals, the pair of heat sealed portions being arranged approximately symmetrically relative to a central axis of the flattened cylindrical body. The check valve prevents a fluid in a fluid container from leaking or flowing out of the containers and enables the fluid to be poured not only into the fluid container but also out of the container in a short time and with ease when the check valve is attached to a free side edge or opening of the fluid container. The fluid container is composed of a synthetic resin sheet or film which is made independently from the check valve. The container may contain a liquid and/or a gas.
2. Description of the Prior Art
Various check valves for synthetic resin film containers having a function of automatically closing a fluid inlet and outlet portion thereof have been proposed. These conventional check valves comprise a simple combination of linear heat sealed portions which form valve flaps therebetween and which are not provided with fluid returning segments. As a result, these check valves have a drawback of not being tight contact with each other this, fluid sealing performance is degraded. In addition, as the size of the valve flaps are increased, the less time is required to pour fluid into or out of the container, thus improving the efficiency of a fluid inlet and outlet passage of the check valve. However, as the size of the valve flaps increases, their sealing performance is remarkably lowered, which deteriorates the reliability and safety of the check valve. Moreover, when the sheet or film forming the fluid container is made of polyethylene etc. and the contained fluid is a gas such as air etc., the gas in the container is dispersed therefrom and the internal pressure of the container lowers due to the comparatively high permeability of the polyethylene etc. as time elapses, so that the sealing performance between the valve flaps deteriorates.
The leakage of fluid which has been poured into the container and contained therein is a serious phenomenon and is the major drawback of the prior art viewed in terms of reliability, safety, accuracy, etc. in the storage, transportation and supply thereof.
As mentioned above, the leakage of fluid which has been poured into and is contained in the container through the check valve is caused by the defective or deteriorated sealing performance of the valve flaps.
The foregoing deteriorated sealing performance of or between the valve flaps originates intrinsically from the process of heat sealing two or more thermoplastic resin films or sheets together to form a check valve. If the check valve is constructed simply of two opposing sheets of a rectangular film having two heat sealed edge portions extending longitudinally on opposite sides thereof, the valve flaps are deemed to be the sheets themselves. Each of the valve flaps (sheets) comprises a central portion and two peripheral portions each disposed between a heat sealed edge portion and a respective end of the central portion. The peripheral portions each terminate at coalescence points of both valve flaps (the point where the valve flaps and the respective heat sealed edge portion are joined).
It is generally believed that both valve flaps are forced into intimate and tight contact against each other over the full width thereof when pressed together by the internal pressure exerted by the fluid contained in the container. However, a close inspection of the two valve flaps shows the formation of fine or narrow gaps between the peripheral portions. The fluid contents in the container leaks out through the fine gaps in the peripheral portions while the central portions remain sealed due to the internal fluid pressure.
The reason for the formation of the fine gaps in the peripheral portions is considered to be the inhomogeneity of the valve flaps thus sealed under pressure. The central portion is uniform in every respect, but the peripheral portions are somewhat different from the central portion in some respects; viz. (1) a cantilever structure, (2) physical or mechanical properties especially as to behavior of elastic deformation due to thermal history induced by heat sealing and (3) elongation to some extent induced by the force of flowing fluid which has spread both flaps apart at the time of charging or discharging the container. In the case where linear seals are included in the check valve, in addition to heat sealing the edge portions, the linear seals are also heat sealed in order to define and to form them, therefore the conditions or circumstances mentioned above are similarly applicable.
Accordingly, if the check valve includes linear or polygonal heat sealed portions, the valve flaps are defined therebetween and comprise central portions which are separated from each other at maximum when the fluid is poured into or out of the container, and peripheral portions which are disposed on both sides of the central portions and which defined by and adjacent to the linear or polygonal heat sealed portions. Since the peripheral portions of the valve flaps are defined and formed by such a linear or polygonal lines, fine gaps are inevitably formed between the front and rear valve flaps at the peripheral portions, particularly at the corners where the polygonal lines make a turn. These fine gaps extending toward the outside make no more than a temporary resistance to the flow of the fluid thus making matters worse.
The fluid does not leak out through the central portion of the check valve where the valve flaps closely contact each other (due to fluid pressure) when the fluid is contained in the container. The fluid leaks out, however, as a result of a capillary action, i.e., a slit effect in the case of liquid, through the peripheral portions of the check valve which have the fine structural gaps therein, and as a result of a pressure gradient in the case of gas caused by the fact that the pressures in the gaps are higher than that outside the check valve (the atmosphere) though it is lower than that inside the check valve.
It is an object of the present invention to completely eliminate fluid leakage through a check valve.